In land mobile communications, multiple radio wave reflections lead to a confluence at a receiver of several signals which all stem from the same signal generated by a transmitter but differ in arrival time, carrier phase and amplitude. This can impair the transmission performance and cause fading or even signal elimination at the receiving station. These so-called multipath effects particularly appear in urban environments, which are at the same time those areas with the highest demand for mobile communication systems.
The high bit rates (&gt;200 kbit/s) of modern digital mobile radio systems cause a significant part of the typical multipath effects to appear as inter-symbol interference (ISI), which can be handled by suitable echo-cancelling equalizers.
A receiver concept in which the digital processing part has been maximized with respect to the analog processing part is disclosed by Paul A. M. Bune "Effective Low Effort Adaptive Equalizers for Digital Mobilophone System", 39th IEEE Vehicular Technology Conference, San Francisco, 1989.
This receiver comprises means for low-pass filtering of the received signal, coherent I/Q sampling and A/D conversion, estimation of the complex channel impulse response (CIR) and filtering by an adaptive matched filter (AMF). If desired, the AMF output can be led directly through a threshold device for reconstruction of the binary signal or can be further processed (equalization, decoding).
Equalizers widely used in telecommunication systems are transversal equalizers and decision-feedback equalizers (DFEs), as these basic structures are very simple and suitable for real-time signal processing, employing either dedicated hardware or programmable digital signal processors (DSPs). The equalizer structure typically comprises a finite impulse response (FIR) filter, eventually combined with a symbol detector (usually a threshold device). The FIR filter tap coefficients have to be adapted to the input impulse response, for which several techniques have been developed and described in the literature. These techniques can roughly be separated into simple but slow, and fast but complex algorithms.
The first type of algorithms includes the zero-forcing algorithm and the mean-square-error (MSE) gradient algorithm, together with their variations. They are very useful for constant or slowly varying dispersive channels such as wired links and fixed radio links, but their performance is sensitive to rapid channel variations which are typical for mobile radio. Their adaption speed can be increased considerably using Kalman or fast Kalman algorithms, but this leads to more complexity and, consequently, to a larger processing effort.
Another strategy for tap adaptation is the computation directly from the channel impulse response, but the methods given in the literature typically involve the computation of either the solution of a set of linear equations or the inversion of a matrix, thus also representing relatively high processing loads when the channel parameters are changing rapidly, as a result, the equalizer therefore has to be updated frequently.